1. Field of the Invention
The present invention relates to a electron beam writing data creating device which can remarkably reduce generation of small graphic data causing deterioration of dimensional accuracy.
2. Description of the Background Art
Due to high fine workability and high controllability, electron beam writing is widely employed for preparing masks which are employed for fabricating semiconductor devices, particularly large-scale integrated circuits (LSIs). Electron beam writing systems can be classified into raster scan and vector scan systems. The raster scan system, which is widely employed for preparing masks in general due to simple device structures and easiness in creation of writing data, has such a disadvantage that the writing speed significantly depends on the minimum grid size (address unit size) for specifying the sizes of the written graphics and the writing positions. In practice, it may be impossible to write graphics in preparation of masks for a 64-Mbit dynamic random access memory (DRAM) or the like requiring small address units by the raster scan system, due to excessively long writing times.
A variable shaping system, which is one of vector scan systems, is recently watched with interest. The variable shaping writing system is adapted to shape an electron beam in correspondence to the sizes of written graphics for irradiating only necessary areas with the electron beam. Thus, the writing speed is increased, and the address unit size can be reduced. Thus, it is considered that the variable shaping system forms the mainstream of electron beam direct writing employed for preparing masks for devices following the 64-M DRAM and development of devices following the 1-G DRAM.
While the variable shaping writing system has the aforementioned characteristics, the processing for creating writing data in this system is disadvantageously complicated and requires a long processing time as compared with the raster scan system. The procedure of creating writing data employed for the variable shaping writing system and the writing procedure are now described.
FIG. 20 illustrates a part of design pattern data 20 for an LSI, which is generally expressed as a polygon (enumeration of vertex coordinates). In order to write the pattern data 20 by a variable shaping electron beam writing system, it is necessary to split the same into basic graphics such as rectangles, squares, triangles and/or trapezoids (hereinafter simply referred to as "trapezoids"). FIGS. 21 and 22 illustrate the pattern data 20 split into trapezoids by horizontally and vertically inserting split lines from vertices of the pattern data 20 respectively.
First, the procedure of actually writing a graphic with the pattern data 20 shown in FIG. 21 is described. When the pattern data of an area 20a shown in FIG. 21 is inputted in an electron beam writing system, an electron beam shaping deflector forms an electron beam having a width Ws1 and a height Hs1.
Then, an electron beam irradiation position specifying deflector moves the beam to an irradiation position (x1, y1), for irradiating a resist film which is applied onto a sample such as a mask substrate or a silicon wafer with the electron beam by a time corresponding to an exposure necessary for photosensitizing the resist film. Then, the pattern data corresponding to an area 20b in FIG. 21 is inputted in the electron beam writing system, which in turn executes electron beam writing system in a similar manner to the above. This processing is successively repeated as to all patterns of the LSI, thereby writing all patterns of the LSI.
Problems in creation of writing data through the aforementioned variable shaping writing system are now described. FIG. 23 shows a resist pattern 21 which is formed on a mask by converting the pattern data 20 shown in FIG. 20 to writing data, writing the pattern on the mask with the writing data, and developing a resist film. Note the pattern dimensional accuracy in a portion of this resist pattern 21 having a width Wm, and consider the factor deteriorating the accuracy. No factor related to pattern formation process conditions such as resist development are taken into consideration here.
When the pattern data 20 shown in FIG. 20 is split along the horizontal split line as shown in FIG. 21, the dimension of the width Wm is influenced only by shaping accuracy of the electron beam which is shaped in response to the graphic of the area 20a shown in FIG. 21, i.e., the dimensional accuracy of the electron beam in the portion corresponding to the width Ws1.
When the pattern data 20 is split along the vertical split line as shown in FIG. 22, however, the dimension of the width Wm is influenced by irradiation positional accuracy of electron beams corresponding to areas 20c and 20d shown in FIG. 22 and the shaping accuracy. Thus, two factors additionally influence the resist pattern dimensional accuracy, as compared with the case of splitting the pattern data 20 along the horizontal split line shown in FIG. 21.
FIGS. 24 and 25 show results obtained by forming a plurality of resist patterns with writing data prepared by different split methods shown in FIGS. 21 and 22 and measuring the dimensions thereof respectively. FIG. 24 shows the result in case of employing the writing data shown in FIG. 21 for forming the resist pattern of the portion having the width Wm by one-shot electron beam irradiation. FIG. 25 shows the result in case of employing the writing data shown in FIG. 22 for forming the resist pattern of the portion having the width Wm by two-shot electron beam irradiation in different positions.
As shown in FIGS. 24 and 25, the dimensional dispersion is about .+-.0.025 .mu.m in case of forming the resist pattern by one-shot electron beam irradiation, while that in case of forming the pattern by two-shot electron beam irradiation is about .+-.0.075 .mu.m. Namely, it is understood that the dispersion of the pattern dimensions, i.e., the dimensional accuracy is deteriorated by the two-shot electron beam irradiation. This is one of the problems in the variable shaping writing data creation.
When a resist pattern is formed by irradiation with an electron beam of two or more shots on different positions and an edge of the resist pattern is formed with a small-sized shot among the plurality of shots, the dimensional accuracy of the formed resist pattern is further deteriorated. This problem is described with reference to FIGS. 26A, 26B, 27A and 27B.
A graph shown in FIG. 26B illustrates the intensity distribution of an electron beam which is shaped as shown in FIG. 26A. The intensity distribution of this electron beam is not completely rectangular, but spread on edge portions. The slopes (hereinafter referred to as "beam sharpness") of the intensity distribution at the edges vary with the size of the shaped beam. In general, Coulomb repulsion in the electron beam is increased as the size of the shaped beam is increased, leading to reduction of the beam sharpness and dulling of the intensity distribution on the edges.
On the other hand, FIG. 27B illustrates intensity distribution of an electron beam of two shots shown in FIG. 27A for forming a pattern of the same size as that shown in FIG. 26A with a small-sized left shot. As shown in FIGS. 27A and 27B, the intensity distribution on the left edge is different from that shown in FIG. 26B, leading to difference between the dimensions of the formed resist patterns.
FIG. 28 shows a result obtained by forming a plurality of resist patterns with two-shot electron beam irradiation including a small graphic as shown in FIGS. 27A and 27B and measuring resist dimensions. As compared with the result (FIG. 24) in case of writing the graphic with one-shot beam irradiation, dimensional dispersion is increased while difference results in the average value. This difference of the average value results from the electron beam irradiation of the small graphic.
The size of such a small graphic exhibiting remarkable deterioration of the dimensional accuracy extremely depends on the employed electron beam writing system, the type of the resist material, the pattern formation process method and the conditions, while the length of a side defining either the width or the height of the graphic data is not more than 0.5 .mu.m, in general.
A writing data processing flow of the conventional variable shaping electron beam writing system is now described. FIG. 29 is a block diagram schematically showing the structure of a conventional electron beam writing data creating device 1.
Referring to FIG. 29, a graphic data processing area split means 2 first splits design layout data inputted in the electron beam writing data creating device 1 into graphic data processing area units of arbitrary dimensions. This operation is adapted to split a chip area of an LSI including enormous graphic data into graphic data processing areas thereby reducing the number of simultaneously processed graphic data and speeding up the processing, while processing the data within the limit of the processing capacity in the system structure (particularly that of a memory area) of the writing data creating device 1. In the graphic data processing area split processing, redundant areas are provided not to cut graphics extending across boundaries between the graphic data processing areas on the boundaries. Namely, areas including the redundant areas define the graphic data processing areas at this point of time. This processing is necessary for splitting the graphic data not to generate small graphics on the boundaries between the graphic data processing areas. The spaces between the boundaries of the redundant areas and the graphic data processing areas may have values sufficiently larger than the sizes of the intended small graphics.
Then, an overlap removal/tone reversal means 4 of the writing data creating device 1 removes overlap between the graphics. This processing, which is adapted to prevent double electron beam irradiation, is requisite for writing data creation of the variable shaping electron beam writing system. In case of irradiating a portion other than the areas inputted in design with the electron beam, tone reversal processing is necessary for generating a graphic in an area having no graphic. Overlap of graphics is removed from portions other than the redundant areas at this point of time. While various methods may be employed for the aforementioned overlap removal and tone reversal processing, a slab method is generally employed. Overlap removal processing employing the slab method is described with reference to FIGS. 30A and 30B.
Referring to FIG. 30A, split lines are formed on the overall surface of the processed area to extend from respective vertices of respective graphics in a fixed direction (horizontal or vertical direction), for splitting the processed area into long rectangular areas (slabs). Then, the graphics are split along the boundaries between the slabs. Referring to FIG. 30A, the processed area is horizontally split.
Then, the sides of the split graphics are directed, thereby defining vectors. The manner of directing is decided depending on whether the vertices are traced clockwise or anticlockwise along the sides. Then, values (hereinafter referred to as "vector direction values") indicating the directions of the vectors in the slabs are provided to the respective vectors. When the slabs are horizontally formed as shown in FIG. 30A, for example, each vector directed from the lower side to the upper side of the vector is provided with a value 1, and that directed from the upper side to the lower side is provided with a value -1. FIG. 30A shows this state in a slab 5. In this case, the vertices of the graphics are traced clockwise.
A method of deleting a portion having overlapping graphics is now described. First, all vectors included in each slab are sorted with coordinate values of contact points between the vectors and the lower side of the slab. Then, the vector direction values provided to the vectors are successively accumulated from the left side when the slab is horizontally formed, for searching for a vector bringing an accumulation result "0" of the vector direction values. A basic graphic is formed by the vector for starting the addition and that bringing the accumulation result "0" . This processing is described with reference to the slab 5 shown in FIG. 30A. The accumulation is started from the vector on the left end, such that the accumulation result of the vector direction values is 2 on the second vector, 1 on the third vector and zero on the fourth vector. Thus, a graphic is formed by the leftmost and fourth vectors in the slab 5, while the second and third vectors are deleted as unnecessary vectors. FIG. 30B shows the result of the processing. The aforementioned processing is performed on the vectors in all slabs, thereby removing overlap of all graphics. It is understood that all data are split into basic graphics as a result of the processing through this method. This method is suitable for processing a large quantity of graphic data, due to a high processing speed.
Then, a redundant part removal means 5 splits graphics included in the redundant areas of the graphic data processing areas not to generate small graphics on the boundaries between the graphic data processing areas, thereby removing overlap of graphics on the redundant areas. Thus, overlap of graphics is removed in all LSI chip areas.
In addition to the aforementioned functions, the writing data creating device 1 comprises a writing field boundary split means 6 having a function of splitting the processed area to areas (hereinafter referred to as "writing field areas") on which the electron beam writing system can write graphics only by deflection of an electron beam, a data formatting means 7 for formatting the data in a data structure which can be inputted in a desired writing system, and the like. In the variable shaping electron beam writing system referred to this time, the sizes of the writing field areas can be set at arbitrary values of not more than 2.5 mm in general.
However, the aforementioned conventional electron beam writing data creating device 1 has the following problems, which are now described with reference to FIGS. 31 to 55:
First, redundant part removal processing is described with reference to FIG. 31. FIG. 31 is a flow chart of the redundant part removal processing. Small graphic dimensions are defined as input parameters, and positional relations between boundaries of graphic data processing areas and graphic data are compared with each other as to graphic data in graphic data processing areas including redundant areas. The aforementioned flow chart is now described with reference to examples shown in FIGS. 32A to 49.
Referring to FIGS. 32A to 32C, a solid line area defined by W1 and W2 is a graphic data processing area 8, broken lines show a boundary of a redundant area 9, and an area defined by P1 and P2 is a graphic data area (hereinafter referred to as "graphic area") 22. The graphic area is defined as a rectangular area which is circumscribed with graphic data. Rectangular graphic data matches with a graphic area, while an area 22' shown in FIG. 33 defines a graphic area in case of trapezoidal graphic data.
As shown in FIG. 31, the aforementioned small graphic dimensions and basic graphic data are inputted (steps S1 and S2), the graphic data processing area 8 is read (step S3), the graphic data are sequentially read (step S4), positional relations between the graphic data processing area boundary (left, right, lower and upper sides) and the graphic area 22 are compared with each other (steps S5 to S8), and boundary processing is performed when the graphic area 22 extends across any graphic data processing area boundary (steps S10 to S17). This boundary processing is described later in detail.
The graphic data is not outputted when the graphic area 22 is present between the boundary of the redundant area 9 and the graphic data processing area 8 as shown in FIG. 32B, while the graphic data is outputted as such when the graphic area 22 is present in the graphic data processing area 8 as shown in FIG. 32C (step S9).
The boundary processing is now described. Each graphic data processing area 8 has the redundant area 9, and adjacent graphic data processing areas 8 to hold a graphic data group in the redundant area 9. Therefore, overlap removal processing can be independently performed on the graphic data in the redundant area 9 for every graphic data processing area 8 without generating small graphics by deciding a certain procedure.
An outline of the aforementioned boundary processing is now described with reference to FIGS. 34 to 49. FIG. 35 is a flow chart of the boundary processing on the graphic area 22 extending across the left side of the graphic data processing area 8. Referring to FIG. 35, symbol .epsilon. represents an inputted small graphic dimensional value, and the remaining symbols in this flow chart correspond to those in FIG. 34.
In this boundary processing, four types of graphic areas 22a, 22b, 22c and 22d in total shown in FIG. 36 are processed. FIG. 37 shows a result of the boundary processing in FIG. 36. Referring to FIG. 36, the width of graphic data on a left portion is smaller than the small graphic dimensional value .epsilon. in the graphic area 22a, in case of splitting the graphic data along a graphic data processing area boundary. In the graphic area 22b, the width of graphic data on a right portion which is split similarly to the graphic area 22a is smaller than the small graphic dimensional value .epsilon.. In the graphic area 22c, both of widths of graphic data split similarly to the graphic area 22a are larger than the small graphic dimensional value .epsilon.. In the graphic area 22d, both of widths of graphic data split similarly to the graphic area 22a are smaller than the small graphic dimensional value .epsilon..
In processing which is common to all sides of the graphic data processing area boundary, no small graphic is generated following splitting along the graphic data processing area boundary (the graphic area 22c shown in FIG. 36). In this case, the graphic is split along the graphic data processing area boundary (steps S1 to S3 in FIG. 35), for outputting only graphic data (graphic area 22c' in FIG. 37) remaining in the graphic data processing area 8, as shown in FIG. 37 (step S4 in FIG. 35). If either one of graphics split on the graphic data processing area 8 is a small graphic (the graphic area 22a or 22b in FIG. 36) and the graphic in the graphic data processing area 8 is small (the graphic area 22b in FIG. 36), the graphics are not outputted, as shown in FIG. 37 (step S1 in FIG. 35). When a graphic outside the graphic data processing area 8 is small (the graphic area 22a in FIG. 36), this graphic area is not split along the graphic data processing area boundary but outputted as such, as shown in FIG. 37 (steps S1, S2 and S4 in FIG. 35). If both of the split graphics are small inside and outside the graphic data processing area 8, as in the graphic area 22d in FIG. 36, this graphic area 22d is not split. In this case, outputted sides are decided. Namely, a graphic such as the aforementioned graphic area 22d is outputted on the right and upper sides of the graphic data processing area boundary, and not outputted on the left and lower sides.
The aforementioned processing is performed on the right, upper and lower sides of the graphic data processing area 8 respectively. FIGS. 38 to 41 show the processing on the right side of the graphic data processing area 8, FIGS. 42 to 45 show that on the upper side of the graphic data processing area 8, and FIGS. 46 to 49 show that on the lower side of the graphic data processing area 8 respectively. Each processing is similar to that on the left side, and hence redundant description is omitted.
The aforementioned boundary processing is performed on the premise that both of graphics in the redundant area 9 are identical to each other in the adjacent graphic data processing areas 8. If graphic data including oblique lines is inputted, however, different split lines enter graphic data in each redundant area 9 in each graphic data processing area 8 although contours of the graphic data in the redundant area 9 appear identical to each other in adjacent graphic data processing areas 8, and hence the graphic data may not be grasped as that of a common graphic by the adjacent graphic data processing area 8, leading to improper boundary processing. Such case is described in more concrete terms with reference to FIGS. 50A to 55.
FIG. 50A shows a kind of design layout data 23 for an LSI. First, the graphic data processing area split means 3 splits the design layout data 23 including a redundant area 9, as shown in FIG. 50B. Thereafter overlap removal processing is performed every graphic data processing area 8. FIG. 50C shows a part of FIG. 50B in an enlarged manner. Namely, FIG. 50C shows first and second adjacent graphic data processing areas 8a and 8b, first and second redundant areas 9a and 9b provided for the first and second graphic data processing areas 8a and 8b, and pattern data 24.
FIG. 51 shows only the pattern data 24 extending across the boundary between the first and second graphic data processing areas 8a and 8b in an extracted manner. FIGS. 52A and 52B show the first and second graphic data processing areas 8a and 8b shown in FIG. 51 in a separated state. In the stage shown in FIGS. 52A and 52B, the pattern data 24 is already split into basic graphics, so that the first and second graphic data processing areas 8a and 8b hold first and second portions 24a and 24b of the pattern data 24 respectively. Boundary processing is performed in the respective graphic data processing areas 8a and 8b on the basis of the aforementioned boundary processing flow chart. Consequently, the first and second portions 24a and 24b become 24a' and 24b' respectively, as shown in FIGS. 53A and 53B. FIGS. 54A and 54B show further basic graphics split from the states shown in FIGS. 53A and 53B respectively. After such splitting into the basic graphics, first and second portions 24a" and 24b" are connected with each other again, as shown in FIG. 55. Consequently, pattern data 24' is obtained. However, this pattern data 24' is deficient, as shown in FIG. 55. Such deficiency of the graphic data disadvantageously results in deterioration of the dimensional accuracy of the writing pattern.